Field of the Invention
The present invention relates to a brush charger for charging a latent image carrier such as a photosensitive drum and, and an image forming apparatus using this brush charger. More particularly, this invention relates to a brush charger which causes a conductive brush to contact a latent image carrier to charge it, and an image forming apparatus using this brush charger.
Description of the Related Art
Image forming apparatuses, such as a copying machine, a printer and a facsimile, employ a latent image forming type recording apparatus like an electrophotographing apparatus, due to a recent demand for image recording on normal sheets of paper. According to this image forming principle, after a photosensitive drum as a latent image carrier is precharged, the photosensitive drum is exposed to a light image to have an electrostatic latent image formed thereon.
This electrostatic latent image is developed by a developing unit so that a toner image is formed on the photosensitive drum. This toner image is then transferred onto a sheet of paper.
To charge the photosensitive drum, a charger whose function is based on corona discharge, such as a corotron, has been used. The corona discharge produces ozone, which is harmful to human bodies, and deteriorates the photosensitive drum. In this respect, there is a demand for a charger which does not use corona discharge, and a brush charger has been proposed as such charging means. The brush charger causes a voltage-applied conductive brush to contact a photosensitive body to thereby charge the photosensitive body.
A rotary type brush charger will be described below. As shown in FIG. 1A, a rotary brush charger 41 has a rotary brush having a conductive brush 411 provided around a rotary shaft 410. This rotary shaft 410 is applied with a voltage by a voltage source 412. A photosensitive drum 40 has a photosensitive layer 400 provided on a conductive base 401. The conductive brush 411 is arranged to face the photosensitive layer 400 in such a way that its tip comes in contact with the photosensitive layer 400. As shown in FIG. 1B, the photosensitive layer 400 of the photosensitive drum 40 is charged with the voltage applied to the rotary shaft 410 of the brush charger 41. Clogging of a foreign matter such as a residue toner between the photosensitive drum 40 and the conductive brush 411 can be prevented by rotating the rotary shaft 410 of this rotary brush charger 41 in the same direction as the rotational direction of the photosensitive drum 40 as indicated by the arrow.
As this brush charger 41 charges the photosensitive drum 40 without using corona discharge, no harmful ozone will be produced, thus contributing to accomplishing an excellent image forming apparatus from the viewpoint of environmental protection.
As mentioned above, the brush charger is based on the principle that the conductive brush 411 contacts the photosensitive drum 40 to charge the drum 40. To surely provide the proper contact, the conductive brush 411 should contact the photosensitive drum 40 with a certain degree of pressure. This pressure is likely to bend the bristles of the conductive brush 411 at both open ends as shown in FIG. 2. In other words, at both ends of the conductive brush 411, the conductive fibers of this brush 411 are pushed in the axial direction of the photosensitive drum 40 and directly contact the conductive base 401 of the photosensitive drum 40 at both ends thereof, or non-photosensitive regions 402 of the photosensitive drum 40 at both ends thereof where there is no photosensitive layer 400 formed. When this occurs, a current concentrates on those pushed conductive fibers of the conductive brush 411, causing a current leak. This leak causes the charged potentials at both widthwise ends of the photosensitive layer 400 of the photosensitive drum 40 to become unstable. The potential levels of those regions of the photosensitive drum 40 therefore become lower so that a toner will stick there, staining the print background portion. Further, the pushed bristles of the conductive brush 411 may be damaged by the concentrated current.
To overcome the above shortcoming, the photosensitive layer 400 of the photosensitive drum 40 may be formed wider to prevent the conductive fibers of the conductive brush 411 from contacting the conductive base 401. As the widened portion is hardly charged, however, a toner stick there, staining both ends of a sheet on which a latent image is transferred.